1. Field of the Invention
The invention relates to molten metal pumps and, more particularly, to a molten metal pump having an attached filter.
2. Description of the Prior Art
In the course of processing molten metal, it often is necessary to transfer the molten metal from one vessel to another or to circulate the molten metal within a given vessel. Molten metal pumps commonly are used for these purposes. The pumps also can be used for other purposes, such as to inject purifying gases into the molten metal being pumped. A variety of pumps as described are available from Metaullics Systems, 31935 Aurora Road, Solon, Ohio 44139, under the Model designation M12 et al.
In the particular case where molten metal is melted in a reverberatory furnace, the furnace is provided with an external well in which a pump is disposed. The pump draws molten metal from the furnace and either circulates the molten metal through the external well (from which it is re-introduced into the furnace), or it transfers the molten metal out of the well to another vessel. Typically, a thermocouple will be placed in the well in order to feed back the temperature of the molten metal to the furnace for appropriate control of the furnace.
A problem with the foregoing arrangement is that foreign material such as dross, solids, or semi-solids (hereinafter referred to as "particles") contained in the well can be drawn into the molten metal pump. If large particles are drawn into the pump, the pump can be jammed, causing catastrophic failure of the pump. Even if catastrophic failure does not occur, the particles can degrade the performance of the pump or negatively affect the quality of a casting made from the molten metal.
In addition to the problems posed by drawing large particles into the pump, a problem also exists with respect to drawing small particles, on the order of 100 microns or less, into the pump. Although small particles cannot cause catastrophic failure of the pump, they still can negatively affect the quality of a casting made from the molten metal. There is a need to positively filter the molten metal prior to using the molten metal in a casting process. While so-called "downstream" filters have been used to filter the molten metal prior to its introduction into a mold, the filtration burden imposed on downstream filters will be lessened if the molten metal has been fine-filtered at some upstream location. In view of the drawbacks associated with unfiltered molten metal pumps, it has become desirable to attempt to remove particles from the molten metal prior to passage of the molten metal through the pump.
One approach that has been attempted is a so-called gate filter. A gate filter is a porous barrier that is interposed between the furnace and the external well immediately upstream of the pump. In theory, a gate filter will remove particles being circulated out of the furnace, thereby avoiding ingestion of those particles into the pump. In practice, several difficulties have arisen. First, it has been found difficult to install the filter, in part because a frame must be provided for the filter at the junction between the furnace and the well. Second, the filter tends to be lifted by the molten metal, thereby permitting particles to flow into the well underneath the raised filter. Third, a thermal gradient can exist in the metal across the filter from the "hot" side to the "cold" side. The temperature of the molten metal in the well can be lower than the temperature in the furnace on the order of 50.degree.-752.degree. F. Because the temperature sensor for the furnace often is located in the well, the lowering of the temperature of the molten metal in the well causes the control system for the furnace to unnecessarily activate the combustion system for the furnace. In turn, excessive heat generated by the furnace causes even more particles to be formed. An additional problem is that oxides and dross formed in the pump well can be drawn into the pump.
Another approach that has been attempted is to suspend the pump within a liquid-permeable filter basket. In effect, the basket acts as a filter for the pump. A drawback of the basket approach is that it is difficult to properly position the pump relative to the basket. The basket must be rested on, or adjacent to, the floor of the well, and the pump must be properly suspended within the basket. Additionally, the basket must be relatively large in order to extend completely above the upper surface of the molten metal. Because the basket extends out of the molten metal, it must be insulated in some manner in order to minimize heat losses through the upper surface. Also, because the basket is so large, its cost is greater than desired.
One approach that has been effective is disclosed in U.S. Pat. No. 4,940,384, issued Jul. 10, 1990 to L. H. Amra, et al., entitled "Molten Metal Pump with Filter," (hereinafter referred to as the "Pump Filter Patent"), the disclosure of which is incorporated herein by reference. In the Pump Filter Patent, a filter is attached to a base of a pump so as to surround the inlet of the pump. Preferably, the filter is made of a porous, bonded (fired or sintered), refractory substance such as silicon carbide and/or alumina. The surface area of the pump is quite large relative to the inlet area of the pump. Due to the configuration of the filter, a large cavity is created that is defined by the interior of the filter and the bottom surface of the pump.
Due to the configuration of the filter and its relationship to the pump, the filter can have a very low porosity, for example, approximately 35-38 percent. The filter not only filters coarse particles that can ruin the pump, but it also filters fine particles that can negatively affect a casting. The filter can be cleaned easily and, when cleaning no longer is feasible, it can be removed and replaced without difficulty. The compactness of the filter minimizes installation difficulties and it also minimizes the expense of the filter.
Despite the advantages of the filter arrangement disclosed in the Pump Filter Patent, certain problems have not been addressed. One of those problems relates to the strength of the filter and the integrity of the filter-pump attachment. Desirably, the filter would be as strong as possible, and it would be attached to the pump in a manner that not only would enhance the strength of the filter, but it also would provide additional support for the base of the pump. Another problem not addressed by the Pump Filter Patent is the possible removal of an impeller and supporting shaft for purposes of repair or replacement without disturbing the attachment between the filter and the pump. Desirably, the filter would be constructed such that the filter could remain attached to the pump while permitting the impeller and shaft to be removed for purposes of repair or replacement.
In view of the approaches that have been described, there remains a need for an effective technique for filtering molten metal being passed through a molten metal pump. It is hoped that any such technique would be inexpensive, easy to work with, and would avoid the drawbacks and address the noted problems of the approaches described above.